1. Field of the Disclosure
The present disclosure relates to a semiconductor laser diode, and more particularly, to a semiconductor laser diode having a ridge-shaped layer on an active layer.
2. Description of the Related Art
As semiconductor laser diodes require high optical extraction efficiency against applied power, research is being conducted to optimize their structure.
A typical semiconductor laser diode having a p-type electrode contacting the entire surface of a p-type cladding layer has difficulty in operating in a single transverse mode using laser light generated in an active layer. Thus, a ridge-shaped cladding layer is formed on the active layer in order to achieve single transverse mode operation.
FIG. 1 illustrates a path which carriers flow from a p-type electrode 15 to an n-type electrode 17 in a conventional semiconductor laser diode having a ridge. Referring to FIG. 1, the conventional semiconductor laser diode includes a substrate 10, an n-semiconductor layer 11, an n-cladding layer 12, an active layer 13 having a multi-quantum well (MQW) structure, a ridge-shaped p-cladding layer 14, and the p-type electrode 15 formed sequentially on the substrate 10. The conventional semiconductor laser diode further includes the n-type electrode 17 formed on a portion of the n-semiconductor layer 11 where the n-cladding layer 12 is absent. As illustrated in FIG. 1, the conventional semiconductor laser diode with the ridge-shaped p-cladding layer 14 and the p-type electrode 15, formed on the active layer 13 restricts the path along which current is injected into the active layer 13.
By increasing a ridge width in a semiconductor laser diode having a ridge, optical output power may be improved.
However, in the conventional ridged semiconductor laser diode in which the dimension of the p-type electrode 15 is equal to that of the ridge, a maximum output power can be increased by a limited degree and a saturated voltage drop occurs even when the ridge width is increased. This results from current crowding.
FIG. 2 illustrates the carrier density distribution for the conventional ridged semiconductor laser diode of FIG. 1. As illustrated in FIGS. 1 and 2, there is a high density of carriers flowing along a path from a portion 15a of the p-type electrode 15 close to the n-type electrode 17. Also as is evident from FIG. 3, this current crowding leads to a non-uniform carrier density distribution.
In this way, the conventional ridged semiconductor laser diode suffers current crowding on the ridge having the same dimensions as the p-type electrode 15. That is, carrier density is high in a portion of the ridge close to the n-type electrode 17. The current crowding effect becomes more severe as ridge width increases.
FIG. 3 illustrates a change in voltage with respect to a ridge width for the conventional ridged semiconductor laser diode of FIG. 1 when current of 100 mA is applied. As evident from FIG. 3, voltage is not substantially further reduced when the ridge width exceeds a predetermined value, e.g. 4 μm, which is called voltage saturation.
As described above, in the conventional ridged semiconductor laser diode in which the dimensions of the p-type electrode 15 are equal to that of the ridge, a maximum output power cannot be further increased and a saturated voltage drop occurs when a ridge width exceeds a predetermined value due to current crowding. Non-uniform carrier distribution due to current crowding also may cause reliability problems such as degradation due to local heating.